P0420 — Catalyst Efficiency Below Threshold (Chery Context)
Standard meaning: the downstream O2 sensor sees the catalytic converter cleaning less than it should. On a 15-year-old car this often really is a tired cat. On a Chery, the whole Australian fleet is 2023-or-newer — a genuinely worn catalyst is near the bottom of the suspect list, by our reading. That changes where a diagnosis might start; the code alone does not set that order.
What it means on your Chery
| Scenario | Likely story | First move |
|---|---|---|
| Young car, low kilometres | Sensor readings, software calibration, or fuel quality — not the cat itself. | See an authorised Chery dealer and check Chery Australia’s warranty terms for your car: whether a repair is covered depends on the fault and those terms. |
| After sustained wrong fuel | Fouling can depress readings; often recovers. | Our suggestion, not a diagnosis from the code: correct the fuel, drive normally, and see whether the code returns. |
| With misfire codes alongside | Misfires dump unburned fuel into the cat — P0300 is the disease, P0420 the symptom. | By our reading, have the misfire diagnosed first. |
Safe owner checks
- Any misfire code present too? That ordering decides everything.
- Fuel history — grade and station consistency.
- Exhaust smell of rotten eggs after hard driving? Note it for the dealer.
When it’s a workshop job
All of it beyond the checks above — and inside a 7-year warranty there is no rational DIY path: a persistent P0420 on a young engine is precisely the kind of fault the warranty exists for. Document, present, insist on root cause rather than a cleared code.
Code definition per SAE J2012. Context: fleet-age reasoning from our model records; no Chery-specific catalyst failure pattern is documented in our research — stated rather than implied.